Bombesin-related peptides ([gastrin-releasing peptide [GRP], neuromedin B) interact with two distinct receptors (GRP-R, NMB-R) to mediate a number of effects in the gastrointestinal tract (GI), central nervous sytem (CNS) and on growth of normal and neoplastic tissues. Furthermore, two related receptors, a mammalian orphan receptor (BRS- 3), having 60% homology to GRP-R and a novel receptor in amphibians, BB-4-R has been described recently. The aims of this project are to understand the pharmacology, molecular pharmacology, and cell biology of these receptors as well as to develop specific agonists and antagonists that can be used to determine their physiological roles. Investigations being performed include expression of these receptors in stable cell lines that resemble native receptors in their cell biology and pharmacology; investigations using site-directed mutagenesis and receptor chimeras to define receptor structural determinants of ligand selectivity and specificity for agonists and antagonists, pharmacological studies of BN-related peptides to identify selective agonists/antagonists and studies of native cells and transfected cells to define the transduction cascades of these receptors. During the last year the ability of a newly described NMB receptor antagonist PD 168368 to interact with each of the 4 classes of bombesin receptors was investigated (NPET 290,1202,1999). This compound was demonstrated to be highly selective for the NMB-R, to function as a pure antagonist, however, its potential for in vivo utility may be limited by its solubility in aqueous solutions. Using chimeric NMB-R and GRP-R as well as site-directed mutagenesis, the molecular basis for PD 16368 NMB-R selectivity is now being investigated. Preliminary results suggest that in contrast to peptide antagonists, its selectivity is determined by interacting with the transmembrane regions of the receptor. Studies are now in progress to define the exact region and amino acids involved in determining selectivity. - bombesin receptors, gastrin-releasing peptide, neuromedin B, BRS-3